Prepare steamed rice, then allow it to cool down to approximately 60°C (140°F). It's crucial not to let it get too cold or too hot.

In a large, clean pot or the inner pot of a rice cooker, combine the cooled steamed rice and the crumbled rice koji. Mix them thoroughly with clean hands or a spoon, ensuring the koji is evenly distributed throughout the rice.

Pour in the warmed filtered water (around 60°C or 140°F) and stir again until everything is well combined.

Maintain the mixture's temperature between 55°C and 60°C (130°F-140°F) for 6 to 10 hours. This is the fermentation period. You can use a rice cooker on its 'warm' setting (checking temperature frequently), a slow cooker on its lowest setting, or a yogurt maker. Use a food thermometer to monitor the temperature regularly; if it gets too hot, the enzymes will be destroyed; if too cold, fermentation will not occur.

Stir the mixture every 1-2 hours to ensure even fermentation and temperature distribution.

After 6-10 hours, taste the amazake. It should be pleasantly sweet. If it's not sweet enough, continue fermenting for another hour or two, checking periodically.

Once the desired sweetness is achieved, immediately transfer the amazake to an airtight container and refrigerate to halt the fermentation process. Amazake can be stored in the refrigerator for up to one week.